Enhanced Generic Phase-field Model of Irradiation Materials: Fission Gas Bubble Growth Kinetics in Polycrystalline UO2 (original) (raw)

Experiments show that inter-granular and intra-granular gas bubbles have different growth kinetics which results in heterogeneous gas bubble microstructures in irradiated nuclear fuels. A science-based model predicting the heterogeneous microstructure evolution kinetics is desired, which enables one to study the effect of thermodynamic and kinetic properties of the system on gas bubble microstructure evolution kinetics and morphology, improve the understanding of the formation mechanisms of heterogeneous gas bubble microstructure, and provide the microstructure to macroscale approaches to study their impact on thermo-mechanical properties such as thermal conductivity, gas release, volume swelling, and cracking. In a previous report "Mesoscale Benchmark Demonstration, Problem 1: Mesoscale Simulations of Intragranular Fission Gas Bubbles in UO 2 under Post-irradiation Thermal Annealing"[1], a phase-field model was developed to simulate the intra-granular gas bubble evolution in a single crystal during postirradiation thermal annealing. In this work, the phase-field model of intra-granular gas atom and bubble behavior was enhanced by incorporating thermodynamic and kinetic properties at grain boundaries, which can be obtained from atomistic simulations, to simulate fission gas bubble growth kinetics in polycrystalline UO 2 fuels. The model takes into account gas atom and vacancy diffusion, vacancy trapping and emission at defects, gas atom absorption and resolution at gas bubbles, internal pressure in gas bubbles, elastic interaction between defects and gas bubbles, and the difference of thermodynamic and kinetic properties in matrix and grain boundaries. The enhanced phase-field model was used to simulate gas atom segregation at grain boundaries and the effect of interfacial energy and gas mobility on gas bubble morphology and growth kinetics in a bi-crystal UO 2 during post-irradiation thermal annealing. The preliminary results demonstrate that the model can produce the equilibrium thermodynamic properties and the morphology of gas bubbles at grain boundaries for given grain boundary properties. These predictive capabilities are important because the gas bubble growth and interlinkage depend on the local gas atom segregation, gas bubble morphology and the fluxes of gas atoms and vacancies from the grain. Furthermore, we can use the model to identify the evolution mechanisms behind the inter-granular gas bubble growth kinetics together with atomistic simulations and experiments. More validation of the model capability in polycrystalline is underway.